1. Color Image Processing
Source & Courtesy: Longin Jan Latecki
CIS Dept. Temple Univ., Philadelphia

2. Light Light is fundamental for color vision
Unless there is a source of light, there is nothing to see!
What do we see?
We do not see objects, but the light that has been reflected by or transmitted through the objects

3. Light and EM waves Light is an electromagnetic wave
If its wavelength is comprised between 400 and 700 nm (visible spectrum), the wave can be detected by the human eye and is called monochromatic light

4. What is color?
It is an attribute of objects (like texture, shape, smoothness, etc.)
It depends on:
spectral characteristics of the light source(s) (e.g., sunlight) illuminating the objects (relative spectral power distribution(s) SPD)
spectral properties of objects (reflectance)
spectral characteristics of the sensors of the imaging device (e.g., the human eye or a digital camera)

5. Primary and Secondary Colors
Due to the different absorption curves of the cones, colors are seen as variable combinations of the so-called primary colors: red, green, and blue
Their wavelengths were standardized by the CIE in 1931: red=700 nm, green=546.1 nm, and blue=435.8 nm
The primary colors can be added to produce the secondary colors of light, magenta (R+B), cyan (G+B), and yellow (R+G)

6. Colors in computer graphics and vision
How to specify a color?
set of coordinates in a color space
Several Color spaces
Relation to the task/perception
blue for hot water

7. Color Models
The purpose of a color model (or color space or color system) is to facilitate the specification of colors in some standard way
A color model provides a coordinate system and a subspace in it where each color is represented by a single point

8. Color spaces Device based color spaces: Perception based color spaces:
color spaces based on the internal of the device: RGB, CMYK, YCbCr
Perception based color spaces:
color spaces made for interaction: HSV
Conversion between them?

9. Red-Green-Blue Most commonly known color space
used (internally) in every monitor
additive

10. The RGB Color Model
If R,G, and B are represented with 8 bits (24-bit RGB image), the total number of colors is (28 )3=16,777,216

11. Cyan-Magenta-Yellow Used internally in color printers Substractive
Complementary to RGB:
C=1-R
M=1-G
Y=1-B
Also CMYK (blacK)
mostly for printer use

12. CMYK K is for blacK Save on color inks, by using black ink preferably
K = min(C,M,Y)
C = C-K
M = M-K
Y = Y-K

13. The RGB color cube

14. The HSI Color Model
RGB, CMY, and the like are hardware-oriented color spaces (suited for image acquisition and display)
The HSI (Hue, Saturation, Intensity) is a perceptive color space (suited for image description and interpretation)
It allows the decoupling of chromatic signals (H+S) from the intensity signal (I)

15. Brightness, Hue, and Saturation
Brightness is a synonym of intensity
Hue represents the impression related to the dominant wavelength of the color stimulus
Saturation expresses the relative color purity (amount of white light in the color)
Hue and Saturation taken together are called the chromaticity coordinates (polar system)
Matlab conversion function: rgb2hsv

16. Example
Comparison:
CMYK,
RGB,
and HSI

17. Class Y color spaces – similar to HIS
YIQ, YUV, YCbCr…
Used in television sets and videos
Y is luminance
I and Q is chromaticity
BW television sets display only Y
Color TV sets convert to RGB
YUV=PAL, YIQ=NTSC

18. Interests of Class Y
Sometimes you have to use it
video input/output
Makes sense in image compression:
better compression ratio if changing class Y before compression
High bandwidth for Y
Small bandwidth for chromaticity
Lab is fine for that too

19. YCbCr Color Space is used in MPEG video compression standards
Y is luminance
Cb is blue chromaticity
Cr is red chromaticity
Y = 0.257*R *G *B + 16
Cr = 0.439*R *G *B + 128
Cb = *R *G *B + 128
YIQ color space (Matlab conversion function: rgb2ntsc):

20. Thanks…